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DISCUSSION
The results of the present study revealed that in subjects with more than 10 years of meditation experience there was a significant increase in the peak amplitude of the Na wave during meditation (mental chanting of OM) compared to a significant decrease in the Na wave peak amplitude during a control period, spent mentally repeating "one". In contrast, naive subjects had a significant reduction in Na wave peak amplitude during meditation, and a nonsignificant trend of reduction during the control period, as well. Our earlier study (Telles & Desiraju, 1993) on 7 subjects with meditation experience ranging from 5 to 25 years, showed that mentally chanting OM produces a small but consistent reduction in the peak latency of the Nb wave. However, this negative component shows considerable variability, and occurs between 35 and 65 ms, and hence it was mentioned that the change had to be viewed caution. It was also reported that changes in Na and Pa waves were fewer and often and often in opposite directions for different subjects, through there was less intersubject variability in the values.
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a N = 9 b p < .001, t-test for paired data, during to pre.
In the present study there was no attempt to assess the Nb wave since the variability precludes using it as a reliable way of assessing the subtle changes in neural processing, which could be expected during meditation. Instead, we placed emphasis on wave V, Na wave, and Pa wave. The subjects of our earlier study (Telles & Desiraju, 1993), had been practicing meditation for several years, and they had evolved individual methods of focusing the mind for meditation. For example, some subjects would mentally focus on the visual image of the syllable OM as it is written in Sanskrit, while other subjects would allow their thoughts mentally to encompass a limitless expanse (e.g., the sky or the ocean). In the present study, the senior subjects had comparable experience of meditation, but this group attended a 10 day retreat during which they were given lectures and instructions on meditation. During the actual practice all of them were instructed to focus attention on the visual image of OM, while mentally chanting it. One can speculate that subtle differences in the mental processes during meditation, could have explained the intersubject differences in the changes in our earlier study cited above since these differences in MLRs could not be correlated with differences in age, or meditation experience. Similarly, the consistency in the mental processes during meditation among the subjects of the present study could explain why changes in the Na wave during meditation were consistent for the whole group. These changes in the Na wave occurred in both senior and naive subjects (though in opposite directions) during meditation, and during control session conducted on senior subjects. However, wave V and Pa wave did not change. 1t has been postulated that the Na wave may be due to activity at the mesencephalic or diencephalic level (Deiber, Ibanez, Fischer, Perrin, & Mauguiere 1988). Hence, either mentally repeating "one" or meditation on OM appears to alter neural processing at the same level. However, the nature of change is dependent on whether subjects are "senior" or "naive," and occurs in opposite directions during the two sessions in senior subjects, and in the same direction during both sessions in naive subjects. An increase in amplitude of MLRs has been interpreted as an indicator of increased efficiency in activating the neural generator (Woods & Clayworth, 1985). Hence, it appears that meditation in "senior" subjects facilitates neural activity at mesencephalic or thalamic (medial geniculate) level, similar to that noted in subjects practicing consciously regulated yogic breathing or pranayama (Telles, Joseph, Venkatesh, & Desiraju, 1993). In contrast, in naive subjects meditation inhibits neural activity at this level. The same result (i.e., inhibition) is also seen in senior subjects while mentally repeating "one". These findings could be correlated with the facts that (a) "senior" subjects found meditation on OM less effort than mental repetition of "one," whereas (b) "naive" subjects felt the opposite i.e., they felt that both meditation and repeating "one" required effort, which was greater for the former. Hence, the results of the present study may provide objective correlates for the altered mental state during meditation. These results can, perhaps, be correlated with the subjective experiences of the volunteers. |